1. Field of the Invention
The invention relates to a flame detector.
2. Description of the Prior Art
A known flame detector has a radiation sensor sensitive in the ultra-violet and/or visible range of the electro-magnetic spectrum. Such flame detectors are used for the monitoring of the flame in furnaces. Their task is to recognise when the flame is extinguished, without delay if possible. Flame detectors are a key element in the safety concept of the furnace. In order to obtain a high degree of reliability for the flame detector and for the furnace, it is necessary for the flame detector to respond only to the radiation of the flame, and not to be sensitive to parasitic effects. One source of parasitic effects is sparks occurring when the flame is ignited.
Known solutions for avoiding the unwanted detection of ignition sparks are, on the one hand, optical shielding which prevents the radiation of the ignition sparks from reaching the flame detector. On the other hand, flame detectors are used which are sensitive in the infra-red range of the electro-magnetic spectrum, as the portion of radiation of the ignition sparks in this range is insignificant. The disadvantage with these latter flame detectors is that their signal is highly dependent upon the operating conditions of the furnace.
An object of the invention is to provide a flame detector which is provided with a radiation sensor sensitive in the ultra-violet and/or visible range of the electro-magnetic spectrum and the output signal of which is largely insensitive to ignition sparks.
According to the present invention, there is provided a flame detector comprising:
a radiation sensor sensitive in the ultra-violet and/or visible range of the electromagnetic spectrum, to produce a signal U1 representing the radiation from a flame
a first circuit which derives from the signal U1 a first signal U2 which is proportional to the direct voltage portion of the signal U1;
a second circuit which derives from the signal U1 a second signal U3 which is proportional to the alternating voltage portion of the signal U1, and a subtracter which forms an output signal UA of flame detector such that
UA=U2xe2x88x92U3.
The ignition sparks induce an alternating signal in the radiation sensor, which superimposes the direct signal of the flame, to the extent that this is present. The behaviour over time of this alternating signal is relatively constant and stable in the long-term. The invention makes use of this in that it determines the alternating voltage portion of the signal of the radiation sensor, and derives a direct signal from it which is of the same value as the direct voltage portion which the ignition sparks generate in the signal of the radiation sensor. By subtracting this direct signal derived from the alternating voltage portion from the whole direct voltage portion of the signal of the radiation sensor, a signal is consequently produced which represents only the portion originating from the flame.
As the ignition spark generator is operated with mains voltage, the signal produced by the ignition sparks in the flame detector has a frequency spectrum with maxima at the mains frequency and multiples of the mains frequency. There are ignition spark generators of a first type which generate a signal in the flame detector with a distinct maximum at mains frequency, and ignition spark generators of a second type which generate a signal in the flame detector with a distinct maximum at double the mains frequency. According to a further concept of the invention, the alternating voltage portion of the signal of the radiation sensor is therefore derived by means of a filter, the characteristic of which has a transparency higher by a pre-determined factor for double mains frequency than for mains frequency. The signal at the output of the filter then corresponds to the direct voltage portion generated by both ignition spark generators of the first type and ignition spark generators of the second type.